In voice communication, acoustic echo cancellation is a great challenge. Acoustic echo is caused by the fact that the microphone may re-capture the audio signal played out by the loudspeaker (loudspeaker signal or reference signal), and thus the talker on the other side (far-end) will hear his own voice together with the input of the near end.
Delay jitter is a another problem especially when the voice communication is conducted on a packet-switch network, and generally a jitter buffer would be provided for smoothing the delay jitter so as to improve the voice communication experience.